Golf ball manufacturers are constantly attempting to construct golf balls having a desirable combination of good “feel,” distance, and durability. One way in which the properties of a golf ball may be adjusted is through the cover composition and construction of the ball. Traditionally, softer feeling golf balls were obtained by providing an outer cover formed with natural or synthetic balata over a liquid center surrounded by a tensioned elastomeric wound layer. Because of its softness, however, balata is susceptible to cuts or other damage to the cover, and, thus, lacks the durability required to withstand the numerous mis-hits produced by the average golfer.
For this reason, amateur golfers typically prefer a golf ball constructed with a harder, more durable cover material, such as an ionomer or ionomer blend. Covers formed of ionomeric materials generally provide a good combination of distance and durability. Because of the hard ionomer cover, these balls are difficult to cut, but have a very hard “feel”, and a lower spin rate, making these balls more difficult to draw or fade. The differences in the spin rate can be attributed to the differences in the composition and construction of both the cover and the core.
Many attempts have been made, therefore, to produce a golf ball with the control and feel of a balata-covered ball and the durability of an ionomer-covered ball. For example, U.S. Pat. No. 4,274,637 discloses two- and three-piece golf balls having covers completely or partially formed from a cellular polymeric material to improve backspin.
U.S. Pat. No. 5,002,281 discloses a three-piece solid golf ball having an ionomer cover and a solid core consisting of a soft inner core and a hard outer shell, where the difference in the hardness of the two parts of the core is at least 10 on the JIS-C scale.
Similarly, U.S. Pat. No. 4,781,383 discloses a solid, three-piece golf ball, having an ionomer cover and a core with inner and outer layers, where the inner layer has a diameter of 24 to 29 mm and a Shore D hardness of 15 to 30, and the outer layer has a diameter of 36 to 41 and a Shore D hardness of 55 to 65. The percentage of the ball surface which contacts the club face when the ball is struck is 27 to 35 percent.
European Patent Application No. 0 633 043 discloses a solid, three-piece golf ball with an ionomer or balata cover, a center core, and an intermediate layer. The center core has a diameter of at least 29 mm and a specific gravity of less than 1.4. The intermediate layer has a thickness of at least 1 mm, a specific gravity of less than 1.2, and a hardness of at least 85 on the JIS-C scale.
U.S. Pat. No. 5,586,950 discloses a golf ball having a core and a cover for covering the core, the cover comprising two ionomeric layers of an inner layer, with a high stiffness modulus of approximately 3,000 kgf/cm2 (42,000 psi) to 5,500 kgf/cm2 (78,000 psi) and a thickness of 0.5 to 2.5 mm, and an outer layer with a lower stiffness modulus of approximately 1,000 kgf/cm2 (14,000 psi) to 2,500 kgf/cm2 (35,000 psi) and a thickness of 0.5 to 2.5 mm. The base resin of the inner and outer cover layer contains an ionomer neutralized with a zinc ion.
U.S. Pat. No. 5,803,831 discloses a multi-layer golf ball having soft outer cover, preferably having a Shore D hardness of 48 or less, and a thermoplastic inner cover layer, preferably having a Shore D hardness of at least 65. The outer cover preferably includes at least 75 percent of a soft ionomeric neutralized copolymer formed from a polyolefin, an unsaturated carboxylic acid, and a monomer of the acrylate ester class.
U.S. Pat. Nos. 5,984,806 and 6,015,356 are directed to multi-layer golf balls with smaller and lighter cores produced by including metal particles, or other heavy weight filler materials, in the cover compositions. The covers may include an inner layer that includes a material having a flexural modulus of at least about 15,000 psi and a hardness of at least about 60 Shore D and an outer layer that includes a material, such as a blend of high and low ionomers, having a flexural modulus of from about 1,000 to 10,000 psi and a Shore D hardness of about 65 or less.
U.S. Pat. Nos. 5,553,852 and 5,782,707 disclose a three-piece solid golf ball having a center core, an intermediate layer, and a cover. The ionomeric resin intermediate layer can include various fillers. The outer cover layer, preferably a thermoplastic resin base composition, is softer than the intermediate layer. Iron shots and partial swing shots are affected by this cover design.
U.S. Pat. No. 5,919,101 discloses a solid golf ball comprising a core and a cover formed on the core, wherein the cover has a two-layer structure consisting of an inner cover having a flexural modulus of approximately 3,000 kgf/cm2 (42,000 psi) to 7,000 kgf/cm2 (99,000 psi) and an outer cover formed on the inner cover having a flexural modulus of approximately 1,000 kgf/cm2 (14,000 psi) to 2,800 kgf/cm2 (39,000 psi), and wherein the core is formed from a rubber composition comprising 0.05 to 5 parts by weight of an organic sulfide compound, based on 100 parts by weight of a base rubber. The cover layer can be an ionomer resin, a polyamide, or a nylon.
The above references, however, do not provide an ideal cover material for attaining both good feel and durability. One solution may be golf balls and, in particular, cover layers formed of or blended with metallocene-catalyzed polymers, i.e., polymers produced using single-site metallocene catalysts, which produce polymers with a narrow molecular weight distribution and uniform molecular architecture, so that the order and orientation of the monomers in the polymer, and the amount and type of branching is essentially the same in each polymer chain.
The narrow molecular weight distribution and uniform molecular architecture provides metallocene-catalyzed polymers with properties that are not available with conventional polymers, and allow polymers to be produced having unique properties that are specifically tailored to a particular application. The desired molecular weight distribution and the molecular architecture are obtained by the selection of the appropriate metallocene catalyst and polymerization conditions.
Processes for grafting monomers onto polymers and, in particular, polyolefins, are known in the art. European Patent Application No. 0 266 994 discloses a process for grafting ethylenically unsaturated monomers, such as unsaturated carboxylic acids and anhydrides and derivatives thereof, onto copolymers of ethylene. The disclosed process includes the steps of forming an admixture of the copolymer, monomer, and 25 to 3,000 ppm of an organic peroxide having a half-life of about one minute to 120 minutes at 150° C., and mixing the resultant admixture in an extruder at a temperature above the melting point of the copolymer for a period of time at least four times the half-life of the organic peroxide. The resultant grafted copolymer is then extruded into a shaped article.
U.S. Pat. No. 5,106,916 discloses a process for the catalytic grafting of an ethylenically unsaturated monomer onto a copolymer in which the process of EPA 0 266 994 is modified by the addition of a catalyst comprising water and at least one phosphorous-containing compound selected from the group consisting of compounds of formula HPO(OR1)2, phosphite compounds of formula P(OR2)3 and formula (OR3)P—O—R4—O—P(OR5)2, and di-substituted pentaerythritol diphosphites of formula (R6O)P—O2—RPEO2—P(OR7), where O2RPEO2 is the pentaerythritol moiety, and R1–R7 are specified organic functional groups.
Grafted metallocene-catalyzed polymers, which are commercially available, share the advantages of non-grafted metallocene-catalyzed polymers, including a narrow molecular weight distribution and uniform molecular architecture. The addition of functional groups to the metallocene-catalyzed polymers by grafting allows polymers to be produced having properties that are not available with unfunctionalized metallocene-catalyzed polymers or polymers formed without the use of metallocene catalysts.
While different blend combinations of species of one variety of polymer, such as ionomers, have been successfully used in the prior art, different polymers, such as ionomers and balata or other non-ionic polymers have not been successfully blended for use in golf ball covers. In general, prior art blends of polymer components are immiscible, i.e., heterogeneous on a microscopic scale, and incompatible, i.e., heterogeneous on a macroscopic scale, unless strong interactions are present between the polymer components in the mixture, such as those observed between ionomers and polymers containing carboxylic acid groups. In particular, this lack of compatibility exists when an ionomer is blended with a polyolefin homopolymer, copolymer, or terpolymer that does not contain ionic, acidic, basic, or other polar pendant groups, and is not produced with a metallocene catalyst. These mixtures often have poor tensile strength, impact strength, and the like. Hence, golf balls produced from these incompatible mixtures would be expected to have inferior golf ball properties such as poor durability, cut resistance, and so on. In contrast, a compatible blend may be heterogeneous on a microscopic scale, but is homogeneous on a macroscopic scale, and, thus, has useful golf ball properties.
U.S. Pat. Nos. 5,397,840 and 5,516,847 are directed to golf ball covers comprising a blend of copolymers wherein one or more of the copolymers is a nonionic copolymer, such as a copolymer of ethylene and acrylic acid or methacrylic acid or a copolymer of propylene and acrylic acid, and one or more of the copolymers is an ionic copolymer, such as sodium and zinc neutralized copolymers of ethylene and acrylic acid. However, the “ionic copolymers” are defined in U.S. Pat. No. 5,397,840 as copolymers of an α-olefin and a metal salt of an α,β-unsaturated carboxylic acid, and the “non-ionic copolymers” are copolymers or terpolymers containing ethylene or propylene and acrylic or methacrylic acid monomers. Therefore, strong interactions exist between the metal salts of the “ionic copolymers” and the acrylic or methacrylic acid monomers of the “non-ionic copolymers” that allow compatible blends to be formed. These interactions do not exist in prior art blends of ionomers and polymers that are truly non-ionic or nonpolar, in particular, those polymers produced with a process that does not involve the use of a metallocene catalyst.
U.S. Pat. Nos. 4,986,545; 5,098,105; 5,187,013; 5,330,837; and 5,338,610 disclose golf balls having covers comprising blends of ionomers and modified thermoplastic elastomers, where the thermoplastic elastomers are conventional polymers, i.e., polymers polymerized using catalysts other than metallocene catalysts. The modified polymers include maleic anhydride modified ethylene-propylene copolymers, maleic anhydride modified styrenic block copolymers, maleic anhydride modified ethylene-vinyl acetate copolymers, brominated styrene-isobutylene copolymers, amine modified ethylene-propylene rubber, and polar modified para-methylstyrene-isobutylene copolymers. However, blends of ionomers with modified polyolefins are not exemplified.
U.S. Pat. Nos. 5,542,677 and 5,591,803 are directed to golf ball cover compositions containing blends of high or low carboxylic acid based copolymers and ethylene copolymers such as ethylene alkyl acrylates.
U.S. Pat. No. 5,321,089 is directed to a golf ball cover composition comprising a blend of ethylene-methyl acrylate, an ionomer resin, and a compatibilizer.
As shown in U.S. Pat. No. 5,703,166, metallocene-catalyzed polymers and ionomers form compatible blends of useful golf ball properties. However, there is no known prior art disclosure of golf balls incorporating compositions comprising grafted metallocene-catalyzed polymers.
U.S. Pat. No. 5,981,658 discloses golf ball compositions that contain non-ionic olefinic copolymers produced by metallocene catalysts functionalized with a post-polymerization reaction. The disclosed compositions exhibit improved mechanical properties, such as tensile strength and flexural modulus. The polymers disclosed in the '658 patent do not, however, address problems with processability of these types of polymers.
U.S. Pat. No. 5,830,087 discloses a multi-layer golf ball having a coefficient of restitution of at least about 0.78 with a central core, an inner cover layer containing a non-ionomeric polyolefin material, preferably a plastomer, and an outer cover layer including a thermoplastic material. The golf ball of the invention can be configured having an inner cover layer containing a non-ionomeric material, such as a metallocene-catalyzed polyolefin, and the outer cover layer containing ionomer.
It would be advantageous to provide a golf ball having a cover including at least two layers, wherein the outermost layer includes at least one grafted metallocene-catalyzed polymer, and preferably a blend including a metallocene-catalyzed polymer, to allow improved play characteristics with all types of clubs (i.e., high spin with irons and partial shots and low spin off the driver). The present invention provides such a cover.